\begin{tikzpicture}[>=Stealth]
    \draw (0, 0) circle(2);
    \node at (-0.3,-0.3) {$O$};

    \draw (0, 0) -- (2, 0) node[anchor=west] {$A$};
    \draw [rotate=57.3] (0, 0) -- (2, 0) node[anchor=south] {$B$};

    \draw [->] (0.5, 0) arc (0:57.3:0.5);
    \node at (1.2, 0.4) {$1$弧度};

    \node at (1.2, -0.3) {$r$};
    \node at (2.1, 1.0) {$r$};
\end{tikzpicture}
